I average 18 to the gallon with 75% highway driving, on a 5.0 V8 RWD with 245-75R16 tires. Both SOHC trucks I've had also did that, one actually did a bit better than that. I'd imagine the 4.0 OHV is the same in terms of MPGs but I've been wrong before.
*Edit* The way I measure the MPGs in my trucks is I'll fill them up (run pump till it clicks off, no more) and drive anywhere from half to 3/4 of the tank out, and then fill it back up at the same pump until it clicks off again, just like the first time, and then read how many gallons it took to "restore it" and figure it up based upon the trip odometer which I reset at the beginning.